Bio-degradable foamed products

ABSTRACT

The method provided produces a bio-degradable foamed material with qualities of uniformity of mechanical and physical properties throughout the product including a foam thickness of up to one meter and a finished foam surface suitable for packaging applications. The parameters for producing such a product are selected from a range of variables which includes wall thickness, mold material, use of a susceptor and the type and composition of a susceptor, the number and arrangement of magnetrons and mold shape. Complex shapes produced by the process are also disclosed.

This application is a continuation of application PCT/NZ02/00226, theentire contents of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to improvements in the manufacture ofbiodegradable foamed materials. More specifically the patent relates toan improved process of using microwaves to produce biodegradable foamedshapes. The present invention further relates to the equipment andmethod used in producing foamed products.

BACKGROUND ART

The present invention builds on the invention disclosed inPCT/NZ01/00052. In this patent application, a two stage process isdescribed for producing a biodegradable foamed product with improvedpackaging properties including resilience, compressibility and shockabsorption. The entirety of PCT/NZ01/00052 is incorporated herein byreference.

It is an object of the present invention to produce a novelbio-degradable foamed product.

The field of starch based bio-degradable foamed materials is widelydiscussed in the prior art. A variety of products exist that attempt toproduce bio-degradable foamed materials as discussed in PCT/NZ01/00052.

Extruded Starch Foams for Molded Shapes

Patent U.S. Pat. No. 5,730,824 (National Starch) utilizes extrusion toproduce foam panels. These panels are then laminated together to formthick sheets, which can be wire cut to varying size and shapes. Thisprocess has limitations due to the expensive capital equipment requiredfor manufacturing. As a result of the expensive equipment, the methodnecessitates shipping ‘air’ as the product can only be made in centrallocations. In addition the shapes are either very limited or costlybecause they have to be cut out of sheets instead of molded during thefoaming process.

Another example, U.S. Pat. No. 5,801,207 (Novamont) relates to takingfoamed starch pieces, placing them in a bag or within layers of sheetingand molding the pre-expanded peanuts in foam-in-place molds. Thelimitations of this method are that the foamed peanuts used to make themolds are very bulky and take up a lot of store space, and againincrease expense through having to ship ‘air’ to the point of useinstead of sending dense pellets that can be foamed at point of use. Themethod is also a complicated procedure for the end-user, as they have tofill and seal bags of foamed peanuts and then molds the bag to theproduct shape.

From the above it is hence useful to have a process that allows in situfoaming and further, that the equipment is relatively inexpensive andsimple to use.

Microwaved Starch Foams for Molded Shapes

Two main patents, WO9851466 (Ato-Dlo) and U.S. Pat. No. 5,639,518 (NKK),utilize dielectric heating in processing the starch based materials.

In WO9851466 (Ato-Dlo), the dielectric heating does not take intoaccount the changing dielectric properties of the material as it heats,nor the relationship between the rheological properties (for exampleelasticity and viscosity) and the rate of heating. It further doesn'tidentify or address the detrimental effect of vapor condensation on thefoam surface finish when such a process is used.

Patent U.S. Pat. No. 5,639,518 (NKK), utilizes a number of differentelectromagnetic and electroconductive methods for producing foambio-degradable shapes. It does not identify the importance of a rate ofheating profile or specific rheology of the material being heated on thesuccess of foaming thick walled bio-degradable shapes. It further doesnot identify or address the detrimental effect of vapor condensation onthe foam surface finish when utilizing microwave frequency irradiation.

A further patent, WO 02/20238, (Ato B.V.), details a process of steamheating taking 5 minutes to heat, under pressure, to the desiredtemperature range of 185° C. Such a long processing time reducesthroughput significantly for a semi-continuous process.

In addition, the methods described above produce foams with varyingconsistency depending on the shape required and, often without thecombination of uniform physical and mechanical properties. Theseproperties include density, compressibility, resilience and shockabsorption. All of these properties limit the product applications. Itis therefore desirable to have a method of processing that can produce auniform product using equipment that is relatively inexpensive andsimple to use.

Microwave Oven Designs

U.S. Pat. No. 4,908,486 (Nearctic Research Centre) describes a multiplemagnetron microwave oven design where the oven is comprised of a cavityand at least one energy source. The main advantage disclosed of multipleenergy sources is that the uniformity of drying is improved thusavoiding hot spots and cold spots inherent in some designs using onlysingle energy sources. The oven is described as being useful for thedrying of granular food crops including grain, rice, some fruits andbeans. The apparatus does not however give consideration to use forfoaming of materials, an object of the present invention. Further itdoes not consider the influence of adjusting the power density of theenergy sources. The specification does not teach of processing multiplework pieces at any one time and further, does not address the use ofmolds, shapes and objects other than granular materials.

It is hence useful to have an apparatus that addresses aspects such asvariable energy density, and complex multiple work pieces.

Surface Coatings

In attempts to improve the surface texture or colour of microwavedproducts, susceptors have been considered in a number of applications,especially in domestic food applications. Susceptors are typicallymetallic films attached to microwave packages which are used in foodapplications to crisp or brown the food surface.

For foamed bio-degradable materials, where the goal is to produce asoft, smooth finish rather than the crisp texture desired in foodapplications, problems have been experienced surrounding vaporcondensation at the interface between the mold wall and the foamedmaterial. Although this problem is identified for example in U.S. Pat.No. 5,965,080, (NKK), this U.S. patent refers to the problem of vaporbreaking down an insulating layer on conductive molds causing arcing, aprocessing problem specific to the use of conductive molds, rather thanthe effect of vapor condensation on the surface finish of the foam.

A further patent, U.S. Pat. No. 6,241,929 (Akopyan), recognises thatuniformity is affected when the heat flow on the interface between themold and the foamable material is large and teaches that it is necessaryfor the material and the mold to have much the same dielectricproperties. The patent, whilst describing a principle behind vaporcondensation and its effect on uniformity, does not teach of specificprocesses and examples, particularly for biodegradable foamapplications.

It is therefore desirable to have a process utilizing susceptors thatalso produces a uniform product with a smooth surface finish.

Microwave Mold Designs

U.S. Pat. No. 5,965,080 (NKK) teaches of a method of foaming starchusing conductive mold halves and an insulating section between. Bothhalves then have an alternating current applied thus heating andexpanding the material. The importance of having vapor release sectionsis recognised as otherwise it is acknowledged that insulation breakdownoccurs.

This method however has the problem that conductive molds have a limitedrate of heating range as arcing occurs with increased power densities.Uniformity is a further problem with this method in that fringe effectsoccur in corner areas. Further, complex shapes, which include a mix ofthin and thick walled foam, are difficult to make using this method asthe method is limited by arcing that occurs in thin walled areas.

Two alternative mold arrangements have been considered for expandedplastic materials.

U.S. Pat. No. 4,298,324 (Isobox-Barbier) describes a device for moldingexpanded plastic material. The device consists of a press, a mold bodyand resonant cavity combination. Mold surfaces in contact with materialbeing molded are formed from a resin containing carbon black, which hashigh dielectric losses, and the remaining portion of the mold body ismade of a microwave transparent or transmissive material.

U.S. Pat. No. 5,397,225 (Huels) recognises the attributes of gooddimensional accuracy and long serviceable lifetime for molds to formlatex foams with microwaves. Limitations of practicable wall thicknessesof typical microwave transparent materials are discussed as areexposures to fluctuating temperatures. A new material based onpolyphenylene ether with a passivated surface is described.

Whilst both methods describe useful alternatives, the limitations andconstraints found from using bio-degradable materials are notconsidered.

Mold Liners

U.S. Pat. No. 5,508,498 (Invenetics) teaches of a utensil being a matrixmaterial and a microwave absorptive material. The matrix is formed fromsilicone rubber with a ferrite based absorber material. The patentteaches only of use directed towards food applications and does notconsider closed molds or pressure changes that occur within the mold.

U.S. Pat. No. 4,566,804 (CEM) discusses use of a supporting body foranalysing a product where the supporting body is comprised of a matrixmaterial and a microwave absorptive material, evenly dispersed withinthe matrix material, and is characterized by a Curie temperature of120-140° C. The invention is limited to a purpose of analysing thermallysensitive materials for volatile components and does not contemplatefoaming of a low dielectric material like starch resin within anenclosed mold.

U.S. Pat. No. 5,079,397 (Alcan) teaches of at least two regions ofdifferent lossiness in its susceptor materials. Examples of lossysubstances suitable for inclusion in microwave susceptors are disclosedas well as techniques for application.

None of the above patents however account for use of a susceptor-typeproduct with a closed mold for bio-degradable foams. In particular, theydo not address the critical problems of susceptor and closed moldapplications, being the prevention of condensation from of vaporreleased, and the internal pressures that accumulate within a moldduring starch based foaming processes.

Thin Film

A large number of patents refer to the use of thin films as[a]susceptors. For example, U.S. Pat. No. 5,019,681 (Pillsbury) outlinesprior art in the field of thin film susceptors where a thin layer suchas polyester is used as the substrate with a thin metal film depositedon the substrate. U.S. Pat. No. 5,019,681 outlines further problems,specifically directed towards the breakdown of the susceptor duringheating leaving it only suitable for disposable single-use applications.

The prior art whilst helpful does not identify applications requiringand detailing the constraints necessary for successful bio-degradablefoam applications. In particular, the prior art does not address theissues inherent to susceptors used in conjunction with closed molds asdescribed above. Namely, being the prevention of condensation from vaporreleased, while retaining a soft, smooth, surface finish, and theinternal pressures that accumulate within a mold during starch basedfoaming processes.

Other Particulate Options

U.S. Pat. No. 5,294,763 (Minnesota Mining) describes particulatesusceptors. Particulate susceptors can be divided into two categories;electrically continuous (e.g. carbon black) or electricallydiscontinuous (e.g. ferromagnetic particles).

Again the patent does not describe bio-degradable material foamingapplications and hence does not consider the particular problemsassociated with these materials.

It is an object of the current invention to overcome the limitations ofthe methods above.

It is a further object of the present invention to produce a foamedproduct with uniform physical and mechanical properties such as density,compressibility, resilience, shock absorption and surface finish byaddressing the combination of problems with rate of heating, heatingmethod and mold design in combination.

It is a further object of the present invention to produce a foamedproduct that is bio-degradable and relatively inexpensive compared withprevious methods.

It is an object of the present invention to address the foregoingproblems or at least to provide the public with a useful choice.

All references, including any patents or patent applications cited inthis specification are hereby incorporated by reference. No admission ismade that any reference constitutes prior art. The discussion of thereferences states what their authors assert, and the applicants reservethe right to challenge the accuracy and pertinency of the citeddocuments. It will be clearly understood that, although a number ofprior art publications are referred to herein, this reference does notconstitute an admission that any of these documents form part of thecommon general knowledge in the art, in New Zealand or in any othercountry.

It is acknowledged that the term ‘comprise’ may, under varyingjurisdictions, be attributed with either an exclusive or an inclusivemeaning. For the purpose of this specification, and unless otherwisenoted, the term ‘comprise’ shall have an inclusive meaning—i.e. that itwill be taken to mean an inclusion of not only the listed components itdirectly references, but also other non-specified components orelements. This rationale will also be used when the term ‘comprised’ or‘comprising’ is used in relation to one or more steps in a method orprocess.

Further aspects and advantages of the present invention will becomeapparent from the ensuing description which is given by way of exampleonly.

DISCLOSURE OF INVENTION

For the purposes of this invention, a susceptor is defined as an articlewhich contains microwave interactive material that absorbs microwaveenergy, and converts it into thermal energy. A susceptor may take manyforms, including; a thin film; a liner; a surface coating on a mold. Ina further alternative, the mold is the susceptor.

According to one aspect of the present invention there is provided amethod of producing a bio-degradable foamed product with qualities ofuniformity of mechanical and physical properties throughout the productincluding:

-   -   a finished foam thickness of up to 1 meter; and    -   a finished foam surface suitable for packaging applications; the        method including the steps of:        -   (a) placing a bio-degradable raw material that is in a form            ready for foaming into a mold to form a moldable assembly,            wherein the raw material has been processed into a form            ready for foaming;        -   (b) placing at least one mould and material such assembly            into a microwave cavity, wherein the microwave cavity            includes:            -   (i) a selection of one or more magnetrons focused on a                cavity with a total power density of up to 10 W/cm³;                and/or            -   (ii) a selection of one or more magnetrons by                pre-determination of working volume, final product shape                and mold shape;        -   (c) microwaving said at least one moldable assembly to form            a bio-degradable foamed product;    -   characterized in that the mold includes any one or combinations        of:        -   (i) a susceptor or microwave interactive material applied to            at least one internal surface of the mold cavity;        -   (ii) a microwave interactive material impregnated into the            mold material itself;        -   (iii) a mold material that itself acts as a microwave            interactive material to enable the internal surface of the            mold to heat uniformly.

In the preferred embodiment, the invention utilizes domestic strengthmagnetrons thus restricting capital cost of the microwave machinery.Further it is preferred that a number of magnetrons are used incombination. It is understood by the applicant that this has the effectof improving the uniformity of the final product as well as reducing theapparatus expense. By way of example a 15 kW microwave generator has acost over $150,000 whereas a generator made up of 15 standard domestic 1kW magnetrons can be purchased for approximately $25,000.

It is proven by the applicant that multiple work pieces can be used withthe associated apparatus thus enabling the option of batch orsemi-continuous processing of many pieces at once. The subsequentimprovement in throughput is particularly advantageous and it is knownby the applicant that the proposed method will allow for multiple workpieces.

Semi-continuous processing is also envisaged. One example includes thatdescribed in U.S. Pat. No. 4,298,324 whereby a press, a resonant cavityand a mold structure are used. In an alternative a carrousel arrangementis used. In a further example considered by the applicant, a conveyorbelt is used on which the work piece(s) travel along. The piece(s) aremoved under the apparatus and the belt is raised forming a seal with thewalls and ceiling of the microwave device. The seal avoids loss ofmicrowave energy. As each piece finishes microwave processing the beltmoves forward and the next set of work piece(s) enter the microwave. Inan alternative embodiment, conveyors are used to load multiple cavitymolds into and out of a fixed cavity with a side door or doors, ratherthan the bottom floor sealing mechanism. It will be appreciated theother arrangements are also possible for semi-continuous operation.

In the above described method, the bio-degradable raw material is abio-degradable polymer or additive selected from the group including:renewable natural resources and modifications of those; non-naturalpolymerisation of natural monomers or oligmers produced from naturalresources; polymers obtained by biotechnological production and otherbiodegradable polymers such as polyvinyl alcohol (PVA) orpolycaprolactone; and combinations thereof.

Other additives can also be included. Typically these additives areselected from a range of biodegradable plasticizers, nucleating agents,processing aids; and combinations thereof.

Further additives with an application dependent function can also beincluded such as flame retardants, fungus and mold inhibitors, strengthadjusting additives, adhesion promoters, viscosity modifiers, fillersand rodent repellents.

The preferred method for preparing the material for foaming is byextrusion or similar heat and shear generating processes known in theart.

In the preferred embodiment, the foamable material has a moisturecontent of 5 to 30% (w/w). The level of moisture has been found to bemost preferably in the range of 15 to 22% (w/w).

In the preferred embodiment, the resulting product has similarmechanical properties to traditional materials. For example, polystyreneis a non bio-degradable material widely used for packaging. Preferredembodiments have comparable mechanical properties such as shockabsorption and resilience.

According to a further aspect of the present invention, the base moldmaterial is microwave or substantially microwave transparent. Examplesinclude plastics; ceramics; and glass. Preferably, plastics are selectedfrom the group including: polyethylene (UHMWPE); polyacetal; polysulfone(PSU); polypolyetherimide (ULTEM); polyetherketone (PEEK); epoxy resins;polyphenylene ether; polyphenylsulfone (PPSU); and combinations thereof.Preferably, ceramics are selected from the group including gypsum(plaster of paris) and china clay.

In an alternative embodiment, plastic or ceramic mold materials arereinforced with a filler, microballoons, or glass fibres having lowdielectric losses.

For the purposes of this specification, dielectric constant (relativepermittivity) is associated with the electric field energy stored in thematerial. The dielectric constant is the ratio of the permittivity of asubstance to the permittivity of free space. It is an expression of theextent to which a material concentrates electric flux.

Preferably base mold materials used have a dielectric constant of 0 to10 at a frequency of 2.45 GHz and a loss factor of 0 to 0.1 at afrequency of 2.45 GHz. Most preferably materials are used with adielectric constant of between 0 and 4 at a frequency 2.45 GHz and aloss factor of between 0 and 0.01 at a frequency of 2.45 GHz.

Preferably, molds may include vent holes. These holes are positioned andsized according to the material and shape desired. Vent holes have theeffect of allowing air and vapor to be released from the mold and hencetempering and/or removing pressure increases in the mold duringprocessing.

According to a further aspect of the present invention, the mold alsoincludes a susceptor (or is a susceptor itself) capable of absorbing andconverting microwave energy into thermal energy while also transmittingsufficient microwave energy to the pellets.

It is understood by the applicant that the thermal energy generated bythe mold elevates and maintains the mold surface temperature at a levelthat prevents the occurrence of condensation. Condensation in the moldhas an adverse effect on the foam surface finish. By using a susceptorwith appropriate conditions, foam with a smooth and resilient surfacefinish is achieved. The energy transmitted through the mold is at alevel, which allows the required rate of heating of the pellets to beachieved.

In the applicant's experience, an elevated surface temperature also hasthe added advantage in that it aids in mold release. This is thought tobe because the increased surface temperature reduces or eliminatescondensation of steam. Steam typically breaks down the starch surfaceinto a sticky substance thus making removal from molds difficult.

Whilst the exact mechanism is not certain, it is the applicant'sexperience that maintaining or reducing the melt viscosity reduces theresistance to flow across the mold surface, resulting in an improvedformation of the foam shape and hence finish.

It has been the applicant's experience that the temperature of the innermold surface (susceptor) can be designed to reach steady stateconditions in the desired temperature range. This results in the sametemperature conditions being achieved during each molding cycle, thusgiving consistency between production runs.

Further, the mold surface temperature returns to a level where heattransfer from the mold to the pellets does not have an adverse effect onthe pellets in the period between loading of the mold and microwaveheating. Return to a temperature, which is below the point wheresignificant vapor loss or burning of the pellets occurs, allows the moldto be reused.

Preferably the above elements are achieved by use of: a susceptorincluding; a thin film; a liner; or a surface coating on a mold. In afurther alternative, the mold is the susceptor, with microwaveinteractive material dispersed throughout the mold material.

Preferably, the microwave interactive material in the susceptor isselected from the group including: electrically resistive or conductivematerials, for example, a thin film of a metal or alloy such asaluminium; a resistive or semi conductive substance such as carbonblack; graphite; silicon; silicon carbide; metal oxides; sulfides;ferromagnetic materials such as iron or steel or ferromagnetic alloys(stainless steel); ferromagnetic materials such as ferrites; adielectric material such as acetal; and combinations thereof.Preferably, the susceptor is a liner which includes ferrite dispersed insilicone rubber or other resinous polymeric material.

In an alternative embodiment, the mold itself is a susceptor impregnatedwith microwave interactive material selected from the group including:electrically resistive or conductive materials, for example, a thin filmof a metal or alloy such as aluminium; a resistive or semi conductivesubstance such as carbon black; graphite; silicon; silicon carbide;metal oxides; sulfides; ferromagnetic materials such as iron or steel orferromagnetic alloys (stainless steel); ferromagnetic materials such asferrites; a dielectric material such as acetal; and combinationsthereof.

Preferably the mold surface temperature is greater than the melttemperature of the material being foamed and the temperature of thevapor given off during the process. It is the applicant's experiencethat in such an arrangement, foam with a soft, smooth surface finish andlow abrasive characteristics is achieved.

Further embodiments of molds include the ability to have thick foamshapes. A depth of up to 1 metre may be processed using the aboveapparatus combination giving uniform foaming and subsequent mechanicalproperties. It is the applicant's experience that the apparatuscombination can be used to produce a wide variety of complex shapes onlylimited by the shape of the mold and the physical limitations of themicrowave cavity size.

In a further aspect of the present invention, the material to be foamedand the mold are moved within the microwave field during foaming. It isunderstood by the applicant that this movement aids in improving theuniformity of the final foamed product.

In preferred embodiments, the microwave apparatus can be adjusted sothat the rate of heating and the volume expansion of the material can bealtered to obtain a uniform material. Preferred embodiments have theenergy density variable from 0.001 to 10 W/cm³ and a rate of heating of0.1-20° C. per second temperature rise. More preferably the energydensity is variable from 0.001 to 1 W/cm³ and the rate of heating is5-10° C. per second temperature rise.

A preferred frequency of operation for the microwave is from 100 MHz to5 GHz. More preferably, one single frequency is used during processing.Most preferably, the frequency used is 2450 MHz.

A preferred power for the microwave apparatus is up to 100 kW. It isunderstood by the applicant that the power requirement is however onlylimited by either the physical volume of the microwave cavity or themaximum power densities for a given volume.

Preferred embodiments of the microwave process may either utilize themicrowave cycle described in PCT/NZ01/00052 or a single step cycle.

Pressure in the microwave cavity and/or mold in conjunction with rapiddepressurization can also be used to alter the final properties of thearticle such as foam density, shock absorption and finish. Theparameters for such a process are considered in WO/02/20238 wherepressures of up to 50 bar are considered.

Further controls to temperature and humidity can also be applied to themicrowave cavity and/or mold during processing to vary the mechanicaland surface finish characteristics of the material(s). Prior artsuggests that temperatures in the range from 0° C. to 250° C. areuseful.

In the present invention, the microwave equipment and/or process can beadjusted to give a finished foam density from 35 to 100 kg/m³. Morepreferably, this density ranges from 35 to 50 kg/m³. It has been foundthat this density gives desired physical and mechanical characteristicssimilar to that of non bio-degradable equivalent materials.

From the above method it is shown that a product can be produced that isbiodegradable, has similar mechanical properties to equivalentnon-biodegradable materials and has a similar surface finish toalternative products. The process is relatively cheap in capital costand labor cost compared to existing methods. Limitations of existingprocesses including non-uniformity, adequate surface finish and low runbatch operations, are resolved.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects of the present invention will become apparent from theensuing description which is given by way of example only and withreference to the accompanying drawings in which:

FIG. 1 is an isometric view of rectangular block of foam;

FIG. 2 is an isometric view of bottle mold foam and;

FIG. 3 is an isometric view of a simple shaped foam (from a moldmodified to fit a liner);

FIG. 4 is an isometric view of a complex shaped foam;

FIG. 5 is a graph showing the heating profiles used in Example 1;

FIG. 6 is a graph showing the effect of surface temperatures on abrasiveindex as described in Example 2;

FIG. 7 is a graph showing the temperature profile used in Example 6.

BEST MODES FOR CARRYING OUT THE INVENTION

In the preferred embodiment, the invention utilizes a plurality ofstandard domestic magnetrons all concentrated on a cavity containing thematerial to be foamed and a microwave transparent mold.

In all of the following examples, a microwave consisting of twelve,850-watt domestic magnetrons based around a power supply at 2450megahertz is used for foaming the bio-degradable material. The microwavecavity has a volume of approximately 0.4 m³ with a ceiling, walls, andfloor according to known specifications.

EXAMPLE 1

This example investigates the effect of rate of heating on the degree offoam formation and the density of the foam.

A mold is prepared for processing a shaped foam article as shown in FIG.3. The mold volume is approximately 1140 cm³, with a rectangular centralsection to the site the product to be packaged. Multiple vent holes arepresent on the upper surface of the mold. The mold is made of ultra highmolecular weight polyethylene (UHMWPE). No susceptor is used in thisexample.

The material for foaming consists of an extrudate, with a moisturecontent of 22% (w/w) and produced as per PCTJNZ01/00052 with a basematerial consisting of: TABLE 1 Showing the raw material compositionMaterial Wt % tapioca starch 86.75 polyvinyl alcohol 12 Lecithin 1Magnesium silicate 0.25 TOTAL: 100.00125 grams of said material is placed into the mold and placed within themicrowave cavity.

The samples are then microwaved at atmospheric pressure as follows:TABLE 2 Showing the sample microwave conditions used Sample No. PowerLevel Microwave Time 1 1 260 seconds 2 3  80 seconds 3 6  44 seconds 412  24 seconds

A graphical representation of the resulting heating profiles is shown inFIG. 5.

The resultant foam has the following properties: TABLE 3 Showing %formation and foam density results Sample No. % Formation Foam Density[kg/m³] 1  50% 204 2  70% 144 3  90% 103 4 100% 91

It can be seen from the above example that the higher rate of heatingthe better the foam formation achieved. It also shows that the higherthe rate of heating, the lower the foam density achieved.

The higher rates of heating cause higher vapor pressures to be built upwithin the pellet, and hence a higher internal pressure within the mold.The higher internal pressure results in improved formation of the foam,as it is the internal pressure that forces the foam into the shape ofthe mold.

EXAMPLE 2

This example investigates the effect of mold surface temperature on thesurface finish and abrasive index of the foam, where results arecompared with polystyrene and molded pulp alternatives. The abrasiveindex represents the level of abrasion, which may occur between the foamand the product, which it is packaging.

Molds to form a rectangular block as shown in FIG. 1 are prepared forprocessing as follows:

-   -   The mold volume for both molds is approximately 1140 cm³, with a        rectangular central section to site the product to be packaged.        Multiple vent holes are present on the upper surface of the        mold.    -   Mold 1 is made of ultra high molecular weight polyethylene        (UHMWPE) with a wall thickness of 25 mm.    -   Mold 2 is made of acetal with a wall thickness of 15 mm.

Silicone rubber and ferrite liners of varying compositions are used ineach trial TABLE 4 Showing the liner details Liner 1 2 3 Thickness: 1.6mm 5.0 mm 5.0 mm Weight % 40% 40% 60% ferrite:

The material for foaming consists of an extrudate as described inExample 1. Samples of 125 g of pellets are microwaved at atmosphericpressure on power level 12 as follows:

-   -   1. Two trials were completed using UHMWPE molds (mold 1)        microwaved separately, with a microwave processing time of 24        seconds after which time the temperature and abrasive index was        measured.    -   2. One trial was completed using an acetal mold (mold 2) using a        processing time of 46 seconds.    -   3. Mold 1 (UHMWPE) was re-tested using liner 1 (40% ferrite @        1.6 mm) with a microwave processing time of 24 seconds.    -   4. Mold 1 (UHMWPE) was re-tested using liner 2 (40% ferrite @        5.0 mm) with a microwave processing time of 24 seconds.    -   5. Mold 1 (UHMWPE) was re-tested using liner 3 (60% ferrite @        5.0 mm) with a microwave processing time of 24 seconds.

The resultant foam also shown in the graph in FIG. 6 has the followingproperties: TABLE 5 Showing effect of surface temperature on surfacefinish and abrasive index Abrasive Index Scale: 0 → 10 Highly abrasive:10 Low Abrasion: 0 (polystyrene and molded pulp properties are given byway of reference). Surface Sam- Packaging Temperature Abrasion pleMaterial [° C.] Surface Finish Index 1 starch foam 29 Rough, brittle &pitted 10 2 starch foam 45 Rough, brittle & pitted 9.5 3 starch foam 60Rough, brittle & pitted 9 4 starch foam 74 Rough, brittle & pitted 7 5starch foam 80 Smooth, soft but resilient 3 6 starch foam 120 Smooth,soft but resilient 2 7 starch foam 160 Dry, weak & brittle N/A 8Polystyrene N/A 2 9 molded pulp N/A 5From the above results it can be seen that by elevating the surfacetemperature of the mold, the quality of the surface finish of the foamis improved. This is evident in both the recorded observations and theabrasion index measurement. It can also be seen that the surface finishachieved on the starch foam is comparable with that of polystyrene andsuperior to that of molded pulp packaging.

Steam, given off during the process, condenses on the mold walls and thecondensate causes the cellular structure of outer surface of the foam tocollapse. It also causes pitting and the formation of a hard, brittleand abrasive surface finish. If the temperature of the inner moldsurface is elevated, condensation of the steam is prevented and theresulting foam surface finish is highly improved.

EXAMPLE 3

This example investigates the effect of elevation and control of themold surface temperature on the degree of foam formation.

The UHMWPE mold described in Example 2 and the liners described below,were used in this example to complete a total of nine trials. TABLE 6Showing the mold details Liner 1 2 3 Thickness: 1.6 mm 2.5 mm 5.0 mmWeight % ferrite: 40% 40% 40%1. Using an UHMWPE mold and liner 1, three separate loads of 125 g ofpellets were microwaved on power level 12 for 24 seconds.2. Using an UHMWPE mold and liner 2, three separate loads of 125 g, 135g and 145 g of pellets were microwaved on power level 12 for 24 seconds.3. Using an UHMWPE mold and liner 3, three separate loads of 125 g, 135g and 145 g of pellets were microwaved on power level 12 for 24 seconds.

The resultant foam had the following properties: TABLE 7 Showingimproved degree of formation at lower densities through elevation ofsurface temperature Sample Temp [° C.] Density [kg/m³] Formation Liner1, Sample 1 60 105 100% Liner 1, Sample 2 60 95  80% Liner 1, Sample 360 85  70% Liner 2, Sample 4 80 105 100% Liner 2, Sample 5 80 95  90%Liner 2, Sample 6 80 85  80% Liner 3, Sample 7 120 105 100% Liner 3,Sample 8 120 95 100% Liner 4, Sample 9 120 85 100%

It can be seen from this example that a higher surface temperatureresults in full foam formation at a lower density than occurs with alower surface temperature.

EXAMPLE 4

This example demonstrates how foam shapes of both simple and complexgeometries can be processed using the same microwave configuration.

In this example, the material for foaming consists of an extrudate, asdescribed in Example 1. The microwave geometry is maintained the samethroughout the experiment as that of earlier examples. Four differentshaped molds are used as follows: TABLE 8 Showing the Mold details Mold1 2 3 4 Mold Name: Rectangular Bottle mold Simple End Complex End block(as shown in Cap Cap (as shown in FIG. 2) (as shown in (as shown inFIG. 1) FIG. 3) FIG. 4) Material: UHMWPE UHMWPE UHMWPE UHMWPE Wall 25 mm25 mm 25 mm 25 mm thickness: Volume: 0.00145 m³ 0.00114 m³ 0.00127 m³0.00184 m³

The following trials were then completed where each mold was placed intothe microwave cavity individually and treated as follows: TABLE 9Showing the mold, the amount of raw material used and the microwaveconditions Mold Pellet Load Processing Time 1 (Rectangular) 140 g 30seconds 2 (Bottle mold) 115 g 24 seconds 3 (Simple End Cap) 125 g 24seconds 4 (Complex End Cap) 220 g 38 seconds

After each trial, the density of the resulting foamed product wasmeasured and compared. The results were as follows: TABLE 10 Showing thesuccess of molding different shapes using the same microwave generatorequipment Power Process Mass Mass Trial Level Time Pellets Foam % FormedDensity 1 12 30 sec 153 g 132 g 100%   91 g/L 2 12 24 sec 110 g  94 g100% 82.5 g/L 3 12 24 sec 125 g 106 g 100% 83.5 g/L 4 12 38 sec 220 g186 g 100%  101 g/L

The above trial shows that a wide variety of shapes can be processedgiving uniform properties via the same microwave equipment thus reducingcosts associated with microwave equipment modifications and labor costsin manufacturing such pieces.

EXAMPLE 5

This example investigates whether or not various silicone/ferrite linerscome to steady state temperature and if so, at what steady statetemperature is a good surface finish achieved?

In this example, the material for foaming consists of an extrudate, asdescribed in Example 1.

A mold is used as described in Example 1 (UHMWPE).

Three different types of silicone/ferrite liner are trialed as follows:TABLE 11 Showing the Liner details Liner 1 2 3 Thickness: 1.6 mm 2.5 mm5.0 mm Weight % Ferrite: 40% 40% 40%

A sample of 125 g of starch pellets was microwaved in the UHMWPE moldfitted with liner 1 at power level 12 and with a microwave processingtime of 30 seconds. The process was repeated for liners 2 and 3.

The resulting foamed products gave the following properties: TABLE 12Showing the effect of surface temperature on foam finish Number ofSteady State Runs Surface Required to Total Temperature Foam SurfaceAchieve Number Trial [C.] Finish Steady State of Runs Liner 1 60 Hardpitted surface 4 20 Liner 2 120 Smooth soft surface 4 20 finish Liner 3190 Thermal degradation 4 20 of foam, foam surface dry and brittle

As in example 2, elevating the surface temperature of the mold is foundto improve the surface finish of the foam. This example also shows thata ferrite/silicone liner can be designed so that it comes to steadystate in the desired temperature range. Achieving steady state is ofmajor significance as it allows the mold to be used repeatedly withoutdelay in a production environment. If steady state were not achieved,variation of product quality would be experienced and thermal runawaywould be likely.

EXAMPLE 6

This example illustrates how a thin film metal susceptor can be used togenerate sufficient surface heating to prevent condensation and improvethe surface finish on the foam.

In this example, the material for foaming consists of an extrudate, asdescribed in Example 1.

An UHMWPE mold of volume 1140 cm³ is used, laminated with a polyethyleneterephthalate aluminium (A1/PET) film. The aluminium thickness isapproximately 0.02 microns.

A sample of 125 g of starch pellets were placed in the lined mold andmicrowaved at power level 12 with a microwave processing time of 24seconds. The temperature profile for the aluminium/PET film is shown inFIG. 7.

The resulting foamed product gave a surface result with a smooth, soft,but resilient surface. A comparative mold without an aluminium/PET filmyields foam with a rough, brittle and pitted surface.

The example shows that the thin film aluminium heats when exposed tomicrowave energy as the result of an I²R (Ohmic) heating mechanism. Thisheating generates a surface temperature sufficient to preventcondensation and yield foam with an improved surface finish. Films thatgenerate a surface temperature above this range (180° C.) result inbrowning/burning of the foam surface.

EXAMPLE 7

This example investigates the effect of matching the mold surfacetemperature to the melt temperature of the material. It is known that ifno temperature gradient exists then no net transfer of energy can occur.

In this example, the material for foaming consists of an extrudate, asdescribed in Example 1. A mold is used as described in Example 1(UHMWPE).

A sample of 125 g of starch pellets was microwaved in the UHMWPE mold ata microwave power level of 12 with a microwave processing time of 24seconds. A similar experiment was also completed whereby the moldsurface temperature was less than the melt temperature.

The results were as follows: TABLE 13 showing the effect of surfacetemperature against that of the melt temperature Example TemperatureFoam surface Tsurface > Tmelt & Tvapor Soft smooth surface finishTsurface < Tmelt & Tvapor Hard, brittle, pitted surface finishIt can be seen from the above example that where the surface temperatureis less than that of the melt temperature, a poor foam surface finish isachieved.

From the examples it can be seen that a variety of molds and options foraltering the surface finish can be used as required. In particular moldsurface temperature modifiers particularly aid finish. The processproduces a product with comparable qualities to alternativenon-biodegradable products such as polystyrene. Further, the processcost is minimized by utilizing standard domestic magnetrons rather thanvery expensive high power magnetrons.

Aspects of the present invention have been described by way of exampleonly and it should be appreciated that modifications and additions maybe made thereto without departing from the scope thereof as defined inthe appended claims.

1-52. (canceled)
 53. A method of producing a bio-degradable foamedproduct, with qualities of substantial uniformity of mechanical andphysical properties throughout the product, comprising: a finished foamthickness of up to 1 meter; and a finished foam surface suitable forpackaging applications; the method comprising the steps of: (a) placinga bio-degradable raw material, that is in a form that is ready forfoaming into a mold to form a moldable assembly; (b) placing at leastone moldable assembly into a microwave cavity, wherein the microwavecavity is in operative relationship to: (i) at least one magnetronfocused on said cavity with a total power density of up to 10 W/cm³; or(ii) at least one magnetron selected by pre-determination of workingvolume, final product shape and mold shape; (c) irradiating at least onesaid moldable assembly to form a bio-degradable foamed product; whereinsaid mold comprises at least one of: (i) microwave interactive materialdisposed on at least one internal surface of a cavity in said mold; (ii)microwave interactive material impregnated into the mold materialitself; (iii) mold material that itself acts as a microwave interactivematerial sufficient to enable the internal surface of the mold to beheated substantially uniformly under microwave irradiation; and (iv)combinations thereof such as to enable the internal surface of the moldto be heated substantially uniformly.
 54. A method of producing abio-degradable foamed product as claimed in claim 53 which utilizesdomestic strength magnetrons.
 55. A method of producing a biodegradablefoamed product as claimed in claim 53 by semi continuous processingcomprising: (i) moving at least one moldable assembly on a conveyer beltinto under a microwave apparatus; (ii) sealing said microwave apparatusbetween said belt, walls of said microwave apparatus and a ceiling ofsaid microwave apparatus; (iii) commencing microwave processingirradiation; (iv) moving said conveyor bed forward at a speed such thatas each moldable assembly finishes microwave processing, the nextmoldable assembly enters the microwave apparatus.
 56. A method ofproducing a bio-degradable foamed product as claimed in claim 53 by semicontinuous processing comprising: (i) moving at least one moldableassembly on a conveyer belt into a cavity of a microwave apparatus; (ii)sealing said cavity by closing a door thereto; (iii) commencingmicrowave processing; and (iv) as the piece finishes microwaveprocessing, opening the door is and moving the belt forward such thatthe next moldable assembly enters the microwave apparatus cavity.
 57. Amethod of producing a bio-degradable foamed product as claimed in claim53, wherein the bio-degradable raw material is selected from the groupconsisting of: renewable natural resources and modifications of suchnatural resources; non-natural polymerization products of monomersderived from natural resources; non-natural polymerization products ofoligomers derived from natural resources; polymers obtained bybiotechnological production; and bio-degradable polymers selected fromthe group consisting of: polyvinyl alcohol (PVA); polycaprolactone; andcombinations thereof.
 58. A method of producing a bio-degradable foamedproduct as claimed in claim 53, wherein the bio-degradable raw materialadditionally comprises additives selected from the group consisting of:biodegradable plasticizers; nucleating agents; processing aids; andcombinations thereof.
 59. A method of producing a bio-degradable foamedproduct as claimed in claim 53 wherein the method for preparation of thematerial adapted to be foamed comprises a heat and shear generatingprocess.
 60. A method of producing a bio-degradable foamed product asclaimed in claim 53 wherein the method for preparation of the materialadapted to be foamed comprises extrusion.
 61. A method of producing abio-degradable foamed product as claimed in claim 53 wherein thebio-degradable material adapted to be foamed has a moisture content of 5to 30% (w/w).
 62. A method of producing a bio-degradable foamed productas claimed in claim 53 wherein the base mold material is substantiallymicrowave transparent.
 63. A method of producing a bio-degradable foamedproduct as claimed in claim 53, wherein the base mold material isselected from the group consisting of: a plastic; a ceramic; glass; andcombinations thereof.
 64. A method of producing a bio-degradable foamedproduct as claimed in claim 63 wherein said plastic is at least onemember selected from the group consisting of: polyethylene (UHMWPE);polyacetal; polysulfone (PSU); polypolyetherimide (ULTEM);polyetherketone (PEEK); epoxy resins; polyphenylene ether;polyphenylsulfone (PPSU); and combinations thereof.
 65. A method ofproducing a bio-degradable foamed product as claimed in claim 63 whereinsaid ceramic is at least one member selected from the group consistingof gypsum (plaster of paris) and china clay.
 66. A method of producing abio-degradable foamed product as claimed in claim 53, wherein the moldmaterial is reinforced with at least one low dielectric loss materialselected from the group consisting of: a filler material with a lowdielectric loss; micro-balloons; glass fibres; and combinations thereof.67. A method of producing a bio-degradable foamed product as claimed inclaim 53 wherein said mold material has a dielectric constant of 0 to 10at a frequency of 2.45 GHz, and a loss factor of 0 to 0.1 at a frequencyof 2.45 GHz.
 68. A method of producing a biodegradable foamed product asclaimed in claim 53 wherein the mold has vent holes.
 69. A method ofproducing a bio-degradable foamed product as claimed in claim 53 whereinthe microwave interactive material is applied to the mold in a formselected from the group consisting of: a thin film; a liner; a surfacecoating.
 70. A method of producing a bio-degradable foamed product asclaimed in claim 53, wherein the mold acts as a susceptor and microwaveinteractive material is dispersed throughout the mold material.
 71. Amethod of producing a biodegradable foamed product as claimed in claim53 wherein the microwave interactive material is selected from the groupconsisting of: electrically resistive materials; conductive materials;semi-conductive materials; and combinations thereof.
 72. A method ofproducing a bio-degradable foamed product as claimed in claim 71,wherein the microwave interactive material is aluminium.
 73. A method ofproducing a bio-degradable foamed product as claimed in claim 71,wherein the microwave interactive material is selected from the groupconsisting of; carbon black; graphite; silicon; silicon carbide; metaloxides; sulfides; ferromagnetic materials; ferromagnetic materials; adielectric material; and combinations thereof.
 74. A method of producinga bio-degradable foamed product as claimed in claim 53 wherein themicrowave interactive material is ferrite dispersed in silicone rubberor other resinous polymeric material formed into a shape that lines theinterior surfaces of the mold.
 75. A method of producing abio-degradable foamed product as claimed in claims 53 wherein the innermold surface temperature during microwave processing is about 50° C. to190° C. as a consequence of the presence of a microwave susceptor ormicrowave interactive material.
 76. A method of producing abio-degradable foamed product as claimed in claim 53 wherein, understeady state operating conditions, the maximum inner mold surfacetemperature is about 50° C. to 190° C. as a consequence of the presenceof a selected susceptor or microwave interactive material.
 77. A methodof producing a bio-degradable foamed product as claimed in claim 53wherein the surface temperature of the mold is greater than the melttemperature of the material being foamed and of the temperature of anyvapour given off during the process.
 78. A method of producing abio-degradable foamed product as claimed in claim 53 wherein themoldable assembly is moved within the microwave field during foaming.79. A method of producing a bio-degradable foamed product as claimed inclaim 53 wherein the energy density in said microwave apparatus is about0.001 to 10 W/cm³ during processing.
 80. A method of producing abio-degradable foamed product as claimed in claim 53 wherein theinternal rate of heating within the material during foaming iscontrolled to about 0.1-20° C. per second temperature rise.
 81. A methodof producing a bio-degradable foamed product as claimed in claim 53wherein said microwave is operated, during processing, at a frequency ofabout 100 MHz to 5 GHz.
 82. A method of producing a bio-degradablefoamed product as claimed in claim 53 wherein the power for themicrowave apparatus is up to about 100 kW.
 84. A method of producing abio-degradable foamed product as claimed in claim 53 wherein the nominalmicrowave frequency, power level and energy density remain the same fordifferent shaped foamed objects.
 85. A method of producing abio-degradable foamed product as claimed in claim 53 wherein the nominalmicrowave frequency, power level and energy density are adjusted to givea finished foam density of about 35 to 100 kg/m³.
 86. A biodegradablefoamed product with qualities of substantial uniformity of mechanicaland physical properties throughout the product comprising: a finishedfoam thickness of up to 1 meter; and a finished foam surface suitablefor packaging applications; wherein said product is produced by a methodcomprising the steps of: (a) placing a bio-degradable raw material, thatis in a form that is ready for foaming, into a mold to thereby form amoldable assembly; (b) placing at least one mold containing saidmaterial into a microwave cavity, wherein the microwave cavity is inoperative relationship to: (i) at least one magnetron focused on saidcavity with a total power density of up to 10 W/cm³; or (ii) at leastone magnetron selected by pre-determination of working volume, finalproduct shape and mold shape; and (c) irradiating at least one saidmoldable assembly to form a bio-degradable foamed product; wherein saidmold comprises: (i) microwave interactive material disposed on at leastone internal surface of a cavity in said mold; (ii) microwaveinteractive material impregnated into the mold material itself; (iii)mold material that itself acts as a microwave interactive materialsufficient to enable the internal surface of the mold to be heatedsubstantially uniformly under microwave irradiation; and (iv)combinations thereof such as to enable the internal surface of the moldto heat substantially uniformly.
 87. A biodegradable foamed product witha density of about 35 to 100 kg/m³, a smooth surface finish having aresilience that is substantially the same as polystyrene, and with anabrasion index of about 2 and
 4. 88. A biodegradable foamed product asclaimed in claim 87 where the surface finish is achieved by microwaveirradiation of a moldable assembly comprising a mold and biodegradablefoamable material, such that the inner surface of the mold is heated toa predetermined temperature range.
 89. A biodegradable foamed product asclaimed in claim 88 wherein the inner mold surface heats due to thepresence of a microwave interactive material.
 90. A biodegradable foamedarticle as claimed in claim 88 wherein the predetermined temperaturerange is about 50° C. to 190° C.
 91. A biodegradable foamed article asclaimed in claim 87 wherein foaming is achieved with rapiddepressurization in conjunction with microwave heating.
 92. A mold,adapted to be used to used to foam a material into a foam body,comprising a microwave interactive material that, when irradiated bymicrowaves, comprises a mold inner surface having a temperature that isgreater than the melt temperature of said material being foamed in saidmold and greater than the temperature of vapor given off duringmicrowave irradiation; wherein treating said foamable material withmicrowave radiation in said mold is adapted to produce a product havingproperties comprising: a density of about 35 to 100 kg/m³, a smoothsurface finish having a resilience that is substantially the same asthat of polystyrene, and an abrasion index of about 2 to
 4. 93. A moldas claimed in claim 92 further comprising vent holes.
 94. A moldarrangement as claimed in claim 92 wherein the elevation of thetemperature of the inner mold surface is achieved by lining a base moldmaterial with at least one microwave interactive material selected fromthe group consisting of: a thin film; a liner; and a surface coating.95. A mold as claimed in claim 92 wherein, as a consequence ofirradiation with microwave, the mold inner surface temperature reachessteady state operating condition having a maximum surface temperature ofabout 50° C. to 190° C.
 96. A mold arrangement as claimed in claims 91further comprising said properties being achieved through the additionalapplication of elevated pressure in conjunction with microwaveirradiation.
 97. A mold arrangement as claimed in claim 92 furthercomprising said properties being achieved through the additionalapplication of rapid depressurization in conjunction with microwaveirradiation.